WASHINGTON  Economic sanctions in place for decades haven't stopped Iran from pursuing nuclear ambitions the Bush administration says are a cover for a nuclear bomb or, if administration allegations are true, from funding terrorism across the Middle East.

So why would the sanctions announced Thursday -- a big dose of more of the same -- cause Iran to change its behavior?

"It is unlikely to directly affect Iranian behavior on the nuclear side," said Robert Hunter, a Middle East expert and senior adviser at the Rand Corp. "It is a symbolic message to Iran and everyone else that the U.S. is deadly serious."

The United States has had only mixed results using sanctions as a foreign policy tool in the past against countries like Iran, Cuba and North Korea.

Under decades-old U.S. financial sanctions, virtually all trade and investment activities with the government of Iran -- including government-owned banks -- are prohibited. Other sanctions -- such as those Thursday -- have been imposed on Iranian entities that the United States believes are linked to terrorist activities and the proliferation of weapons of mass destruction.

Despite all that, the United States says Iran funnels hundreds of millions of dollars a year to Hezbollah, Hamas and other terrorist groups. The United States has called the country a "central banker of terror."

Tehran also hasn't backed down from its nuclear ambitions.

Iran's defiance of U.N. Security Council demands that it stop uranium enrichment has led to two sets of economic sanctions against Iran. A third, tougher set of U.N. sanctions has stalled amid opposition from China and Russia.

The United States and some of its allies accuse Iran of secretly trying to develop nuclear weapons and have demanded it halt uranium enrichment, a key step in the production of atomic weapons. Iran denies the claim, saying its program is for peaceful purposes, including generating electricity.

Secretary of State Condoleezza Rice, in announcing the new sanctions with Treasury Secretary Henry Paulson, said Washington remains open to a "diplomatic solution."

The latest sanctions against Iran -- the harshest since the takeover of the U.S. embassy in 1979 -- include among other things, targeting Iran's Revolutionary Guard Corps, the country's powerful military wing, as well as the Iran's special operations unit, Quds Force, which is part of the Guard Corps.

Three of Iran's largest banks -- Bank Melli, Bank Mellat and Bank Saderat -- also were targeted. The United States had already moved in 2006 to sever Bank Saderat from the U.S. financial system.

The order also designates companies, including Khatam al-Anbya Construction Co. and Oriental Oil Kish, believed to be owned or controlled by the Revolutionary Guard Corps, as well as military officials and people involved in Iran's ballistic missile programs.

Under the action, any financial assets found in the United States belonging to those named Thursday must be frozen. Americans are also forbidden from doing business with them. Essentially those targeted are frozen out of the United States, the world's largest economy and home to the globe's most influential banking system.

Importantly, the new sanctions can make others outside the United States think twice about having relationships with those blacklisted.

That requires skillful financial diplomacy.

The United States, which has been pressing allies in Europe, Japan and elsewhere, has had only limited success with persuading financial institutions outside this country to voluntarily sever business or scale back business with Iran.

It is a challenge given Iran's standing in the global economy and its position as a major oil supplier. That makes Iran an attractive investment for companies.

For the new U.S. sanctions to be truly effective, the U.S. needs to convince other countries to follow suit.

"So far, however, the administration has presented only the broad outline of a justification for its actions, not the comprehensive details required to be convincing," said Anthony Cordesman of the Center for International and Strategic Studies.

"There has been no mention of how they relate to U.S. efforts to work with Britain, France and Germany or in the context of the U.N," he said.

"There may well be a good, practical case for the new sanctions, but so far, the administration is acting as if there was no world beyond the Beltway," he said, referring to the highway that encircles Washington.

The United States has had only mixed results using sanctions as a foreign policy tool in the past against other countries, including Cuba and North Korea.

Despite sanctions in place for more than 40 years against Cuba, Fidel Castro's communist regime is intact. The broad sanctions against Cuba and those against Iraq before the ouster of Saddam Hussein probably hurt the people of the countries more than they modified governments' behavior, analysts said.

Financial sanctions aimed at stopping the flow of money to Hamas probably generated sympathy for the group in the Arab world and in some ways has made them more popular.

In North Korea, sanctions left the country more financially isolated and prompted international banks to sever ties there.

But there was a price: A financial dispute over millions of dollars in frozen North Korean assets held by an Asian bank sidelined multi-nation nuclear disarmament talks with North Korea. After much negotiating, the dispute was resolved with the once frozen monies returned to North Korea. In turn, North Korea pledged to move ahead on disabling its main nuclear facilities.